Aggression from other dogs a sign that my pup is sick?

This might be a strange question, but has anyone had a dog that got negative reactions from some dogs and later discovered the dog was sick?

Our 16-month-old pup just spent 2.5 weeks over the holidays with a very experienced Rover sitter. She’s cared for hundreds of dogs, has an excellent reputation, and our dog has stayed with her before without issues. This time, though, when we picked her up, the sitter mentioned she was genuinely puzzled by something she observed.

She noticed that our dog triggered reactive responses from some dogs who are normally dog-friendly. She said she hasn’t seen this exact pattern before. At first, she wondered if it was a confidence issue, but changed her mind when one of the dogs reacting negatively was a shy 7-month-old who was too young to be bullying or dominant. She also mentioned that it's a possibility other dogs sensed that something was “off” about our dog, possibly related to illness.

We’re already planning to take her to a veterinary behaviorist for some other weird behavioral quirks we’ve noticed. I’m wondering if this is a legitimate concern to bring up from a medical standpoint, or if it sounds like a stretch. Has anyone experienced something similar?